This invention relates to centrifugal fans and, more particularly, to a centrifugal fan having a mechanism for varying the pitch of the fan blades while the fan is in operation.
Variable air volume (VAV) systems are total building air conditioning supply systems which utilize a large central air-handling unit to deliver heated or cooled primary air to remote building locations. At each such remote building location a terminal box associated with a single room or space and responsive to the temperature in that room or space operates to vary the volume of conditioned air delivered to the space for temperature control purposes. By varying the volume of conditioned air supplied to each room in accordance with local demand conditions, as opposed to mixing both heated and cooled air to control individual room temperatures, significant energy savings are achieved by VAV systems. The large central air-handling unit at the core of a VAV system represents both a significant initial capital cost, and, due to its energy consumption, a primary operational cost to the building owner. As such, improvements in centrifugal fan technology whether relating to fan efficiency or initial cost represent meritorious and significant advances in the art.
Several modulation schemes exist by which the reduction and/or regulation of fan operation can be accomplished so as to reduce fan power consumption when load conditions allow. Among these fan modulations schemes are systems predicated on the use of: (1) discharge dampers, (2) inlet guide vanes, (3) eddy-current clutches, (4) variable speed belt drives, and (5) AC frequency inverters. All of the above-mentioned apparatus in some way relate to the control of a centrifugal fan so as to minimize power consumption by the fan motor. Several deal with the varying of fan speed in accordance with load conditions while others allow for fan operation at constant speeds but at reduced fan loads. The selection of a particular fan modulation scheme for use in a particular VAV application depends upon many factors. Among these factors are system size, fan and system operating conditions, load distribution, fan type (i.e., forward curved, backwardly inclined or airfoil blades), maintenance requirements and equipment space and cost.
The overall goal of fan modulation is to deliver only the required volume of conditioned air based upon local demand conditions for the lowest energy and initial investment costs. The favored and most economical fan modulation scheme when first cost is primarily considered, involves the use of inlet guide vanes on air handlers equipped with forward curved blades to regulate fan load. Inlet guide vane mechanisms, as best exemplified by U.S. Pat. No. 4,177,007 assigned to the assignee of the present invention, are relatively simple yet rugged and cost-efficient apparatus by which fan modulation can be accomplished. Inlet guide vanes modulate fan load by imparting a spin to the air delivered to the fan wheel in the direction of fan wheel rotation. The effect of this spin or pre-swirl is to cause the unloading of the fan blades which in turn decreases the volume of the air delivered by the fan. This, in turn, decreases the horsepower required to drive the fan wheel. While currently the fan modulation method of choice, inlet guide vanes do have drawbacks which detract from their efficiency and attractiveness for use. A primary disadvantage in the use of inlet guide vanes relates to their bulkiness and disposition near or in the inlet of a centrifugal fan. The location of inlet guide vanes at the fan inlet is an impediment to airflow at peak load conditions. At low load conditions, many inlet guide vane mechanisms are "leaky" and allow for the passage of a significant amount of unneeded air into the fan housing. This additional air only adds to the load on the fan yet does not serve any purpose with respect to building climate control. Further, the effect of inlet guide vanes on fan loading decreases as the demand for airflow decreases. Finally, while inlet guide vanes are relatively mechanically simple and reliable, such mechanisms are not integral with the centrifugal fan. That is, the blades and actuating mechanism for inlet guide vane assemblies often extend outward from the fan housing to the extent that they present fan mounting and installation difficulties, particularly in cramped fan rooms and spaces.
One approach to fan modulation, if successfully implemented as has not yet heretofore been accomplished, relates to varying the pitch angle or angle of attack of the fan blades of a centrifugal fan wheel while the fan is in operation. The blade pitch angle varying approach to fan modulation offers all of the advantages of inlet guide vanes while ameliorating some of their disadvantages. The superior power unloading characteristics of variable pitch fans is significant. As blade pitch angle increases in a variable pitch fan, the fan unloads due to a change in the aerodynamic characteristics of the fan wheel. As the fan unloads, a decrease in the volume of air delivered by the fan and of the static pressure of the system results in a corresponding drop in the horsepower required to drive the fan. Thus, in VAV systems fan performance can be optimally matched to system air volume and static pressure requirements so as to minimize system energy consumption at any given system load. Further, variable blade pitch fans offer effective modulation over a wider range of airflows than do inlet guide vane equipped fans. Finally, experiments indicate that variable blade pitch fans consume approximately one-third less energy in operation than do inlet guide vane equipped fans. Though the advantages of fan modulation by blade pitch variation have been recognized, no operable apparatus has been conceived by which the variance of blade pitch in a centrifugal fan is accomplished in a simple, inexpensive and reliable manner while the fan is in operation. In a VAV application wherein constant monitoring, response and control of a building's climate at discrete locations is required, the ability to vary the blade pitch of a centrifugal fan while the fan is in operation is a prerequisite to the use of such a fan modulation technique.
The primary reason for the lack of an operable centrifugal fan blade pitch varying scheme relates to the extremely high operating speeds and centrifugal forces developed on centrifugal fan blades while the fan is in operation. Such speeds and forces can approach 3,000 RPM and 2,400 G's in relatively large fans. A centrifugal fan and the forces developed thereon are entirely different from the forces developed in other types of fans such as vane axial fans. Further, the ability to modulate fan blade position in a centrifugal fan represents an entirely different design and manufacturing problem than the accomplishment of blade movement in a vane axial fan primarily because of the differences in blade location and attachment to the remainder of the rotating fan structure. Early attempts at blade pitch variation in centrifugal fans can be found in U.S. Pat. Nos. 509,143; 1,180,587; 2,361,007; and more recently, 4,139,330. These U.S. patents are to centrifugal fans or blowers which feature adjustable pitch blades. In all of these patents, however, blade pitch is preset while the fan is not operating and is not variable while the fan is in operation. Further, blade pitch in some of the patents noted above is variable only in a discrete stepwise fashion as opposed to variation of blade pitch to any angle over an entire operating range. Such schemes are thus entirely inappropriate for use in VAV systems wherein airflow and fan performance must be continuously monitored an modulated while the system is operating in order to maintain peak system energy efficiency.
Further, schemes are known by which a shape of a centrifugal fan blade is deformed or the position of a blade is affected by the centrifugal forces developed on such blades in operation so as to affect the flow of air into and through a centrifugal fan. Exemplary in this respect are U.S. Pat. Nos. 3,782,853 and 3,901,623. In both of these patents there is no positive control over blade position. Rather, blade position is merely a function of the centrifugal force acting on the blade at a particular fan wheel speed. Finally, the use of an adjustable trailing edge flap in conjunction with a fixed leading edge blade portion for fan modulation purposes is known. As has been specifically recognized, however, such flap actuating mechanisms are both complicated and costly and the forces acting on the trailing edge flap portions and the localized stresses which result are so high as to be unacceptable in high-speed centrifugal fan applications. Therefore, the need exists for apparatus by which the pitch of the blades of a centrifugal fan wheel can be varied such that acceptable stress levels and fan wheel rigidity are maintained in order to prevent unacceptable vibration and fan noise.